When
is even, the DFT summation can be split into sums over the
odd and even indexes of the input signal:

(A.1)

where
and
denote the even-
and odd-indexed samples from
. Thus, the length
DFT is
computable using two length
DFTs. The complex factors
are called
twiddle factors. The splitting
into sums over even and odd time indexes is called decimation in
time. (For decimation in frequency, the inverse DFT of the
spectrum
is split into sums over even and odd bin
numbers
.)